1. Field of the Invention
The present invention relates to a glass plate which is used as a cover glass for display devices, typically cell phones, PDA and small display devices such as touch panels.
2. Discussion of Background
In recent years, for mobile devices such as cell phones and PDA, use of a cover glass (protective glass) for protecting a display and improving appearance, is increasing.
On the other hand, weight reduction and thickness reduction are required for such PDA. Therefore, a cover glass used for protecting a display is also required to be thin. However, if the thickness of the cover glass is made to be thin, the strength is lowered, and if a display device is dropped at a time of using or carrying it, the cover glass itself may sometimes be broken. Therefore, there is a problem that the cover glass cannot accomplish the original object to protect display devices.
In order to solve the above problem, it is conceivable to improve the strength of the cover glass, and as such a method, a method to form a compressive stress layer on a glass surface is commonly known.
As the method to form a compressive stress layer on a glass surface, typical are an air quenching tempering method (physical tempering method) wherein a surface of a glass plate heated to near the softening point is quenched by air cooling or the like and a chemical tempering method wherein alkali metal ions having a small ion radius (typically Li ions or Na ions) on a glass plate surface are exchanged with alkali ions having a larger ion radius (typically K ions) by ion exchange at a temperature lower than the glass transition point.
As mentioned above, the thickness of the cover glass is required to be thin. If the air cooling tempering method is applied to a thin glass plate, the temperature difference between the surface and the inside tends not to arise, and it is thereby difficult to form a compressive stress layer, and the desired property of high strength cannot be obtained. Therefore, a cover glass tempered by the latter chemical tempering method is usually used (Patent Document 1).
Although the chemical tempering method is a method suitable for tempering a thin glass plate, if it is attempted to apply the chemical tempering method to a thinner glass plate, there is a problem of increasing the internal tensile stress generated to take a balance with a high compressive stress on a surface. That is, if the internal tensile stress is high, when a crack deeper than the surface compressive stress layer is formed, the glass will break spontaneously due to a large tensile strength pulling the tip of the crack. In order to reduce the internal tensile stress to suppress this spontaneous breakage of glass, the depth of the surface compressive stress layer may be reduced. However, in such a case, glass becomes extremely weak against breakage or crack, and the desired strength cannot be obtained.
While such problems are considered in the case of the chemical tempering method, as a cover glass for mobile devices, for example a glass plate (hereinafter referred to as commercially available glass plate) having a thickness of 0.75 mm obtained by chemical tempering of a glass (hereinafter referred to as glass A) having a composition of, as represented by mol %, SiO2: 68.4%, Al2O3: 10.6%, Na2O: 11.9%, K2O: 2.3%, MgO: 5.6%, CaO: 0.3%, TiO2: 0.6% and As2O3: 0.3% has been used. The surface compressive stress S and the surface compressive stress layer thickness t of the commercially available glass plate were measured, and as a result, S was 684 MPa, and t was 48 μm.
Here, when a cover glass is produced, glass is usually polished. The size of abrasive grains used at a first stage of the polishing is typically 100 μm, and it is considered that by the polishing with such abrasive grains, microcracks in a depth of 40 μm are formed (Non-Patent Document 1, FIG. 1.18).
As mentioned above, t of the commercially available glass plate, namely 48 μm, is larger than the typical value of the microcrack depth formed on the glass plate, namely. 40 μm, whereby the commercially available glass tends not to be broken (FIG. 1.18 of Non-Patent Document 1).
Patent Document 1: US Application Serial Number 2008/0286548
Non-Patent Document 1: Glass Engineering Handbook, edited by Masayuki Yamane et. al, first edition, Asakura Publishing Co., Ltd., Jul. 5, 1999, p. 397